(A). Field of the Invention
This invention relates to a novel process for preserving hides as well as recovering undenatured collagen and high quality fat from collagen raw materials. The aim is processing of hides and skins with a minimum amount of manual labor, with a minimum amount of waste, and maximum utilization of by-products. The process encompasses adaptation of a tanning machine to carry out (in addition to the usual operations of hide preservation and tanning) removal of fat using a solvent, and dehydration by means of vacuum and microwave energy to obtain soft, dry hides or skins that do not require salt for preservation. The hides and skins may be unhaired or even unhaired and tanned, and the leather brought to a dry "crust" condition using vacuum and microwave energy without being taken out of the tanning machine. The same process allows trimmings and other offal to be degreased, unhaired, and dried to yield fat and undenatured collagen, a valuable by-product.
(B). Prior Art
Tanning drums and hide processors are presently used commercially for the manufacture of leather and for salting of hides and skins; however, all operations are carried out at atmospheric pressure. Because of the low temperature of denaturation of hide collagen, drying is carried out in special dryers only after tanning that increases the denaturation temperature.
Salt curing of hides is presently by far the most common method of preservation of cattlehides to protect them from damage by microorganisms. Salt removes a sufficient amount of water from the hide and drops the water activity sufficiently to inhibit the growth of most albeit not all microorganisms that damage the hide during shipment and storage. Conventional salting operations take roughly a week and involve piling hides between layers of salt using up as much as half a pound of salt per lb. of hide.
In the U. S. most of the hides produced at or near large slaughter houses are salted by a process called brine curing in runways to reduce the time necessary for salt to penetrate the hide. This process utilizes saturated salt solutions and is preceded by hide washing, fleshing, demanuring (using a manually fed machine in the latter two steps), and hide trimming (also by hand). Following brine curing in a runway, and feeding the hides (manually) into a wringer, they are inspected, graded, and piled before shipping. During fleshing the subcutaneous tissues (which may contain up to 50% of fat with the rest comprised of mostly collagen) are removed from the flesh side of the hide in order to enhance diffusion of salt and thereby to shorten the hide curing process to 24 hours. Another reason for prefleshing is the fact that during prolonged storage of unfleshed hides fatty acids are formed which penetrate into the hide corium and form calcium soaps that cause uneven tanning and coloring of hides during subsequent leather manufacture. Prefleshing also permits rendering of the fleshings and trimmings by melting and pressing to obtain a salable fat by-product and a low-priced, denatured protein by-product. The latter is of little value since collagen is denatured at the high temperature used during rendering. Because of the high costs involved, presently most of the hides are not washed and prefleshed before salting or brine curing. And since salted hides may be subject to bacterial attack, bactericides are added in addition to the salt. This makes any by-products recovered from them unusable for food or medical use. Besides, the decomposition of the fat during shipment and storage of salted hides before they arrive in the tannery yields a fat of low grade due to its dirty color and to an unpleasant smell, especially if the hides have been in contact with sulfide, a chemical generally used for unhairing of hides in the tannery. Even if the hides are prefleshed before curing, the operation is very often not satisfactory enough for leather manufacture and, therefore, the tanner often repeats the process of grading, trimming, and fleshing.
The practice of salting of hides is thus a very cost and labor intensive operation involving duplication of effort first to put the salt into the hide at the slaughter house and then to take the salt out again in the tannery. Recovery of proteins from trimmings in the tannery is also prohibitive because of its high moisture content; the product obtained is too expensive compared to products obtained by the simple process of dry heating of other offal from the animal body. Being hydrophilic, water interferes with penetration of hydrophobic organic solvents into the wet hide material and thereby limits the use of solvent extraction of fats from hides.
While providing poor preservation of hide protein and fat, and necessitating addition of biocide, present hide preservation practices are wasteful of collagen, a valuable material, in the form of trimmings, fleshings, and shavings that often contain chrome, a potentially hazardous chemical. Yet to date all attempts at finding a more cost effective means to preserve hides while minimizing pollution by salt and chrome has been unsuccessful despite a multitude of research studies inspired by ever stricter environmental restrictions and the increased cost of disposal of salt as well as of hide and leather offal. In addition to being very costly, conventional air drying of hides, the oldest alternative method of hide preservation, causes oxidation of fat and hardens the hide, making it almost impossible to rehydrate to its original condition.
Heretofore methods of solvent extraction of oils and fats from different raw materials have been published (see Badger, W. L. and Banchero, J. T. , "Fundamentals of Chemical Engineering", McGraw-Hill Book Company, Inc., New York (1955), p. 331). Also, microwave drying under vacuum of hides and leather have been described (see U. S. Pat. No. No. 4856201, and publications by Heidemann E., and Komanowsky M. cited on the Title page above). However, neither of these two processes are economically feasible for hides and leather. The former because animal fat is relatively cheap, and because present technology does not provide an ecologically satisfactory removal of the solvent remaining in the hide after solvent degreasing without denaturing hide collagen. The latter mostly because the microwave equipment recommended requires a separate drying step in which the hides or leathers are manually hung on rod shaped supports or laid on a belt or a plate support device. Even though the latter technique utilizes very large treatment chambers (because of the space needed between each hide to expose it evenly to the microwave energy), it does not ensure uniform drying over the whole area. The construction of the equipment is intricate, and its use restricted only to drying. Furthermore, it is not adaptable to drying hide trimmings, fleshings or other animal by-products containing collagen. Also, without mechanical flexing of hides during drying that is achieved by using the method described in this patent, untanned dry hides are not soft and pliable.